Method and apparatus for producing hollow plastic objects with pressurized gas injection after overflow cut-off

ABSTRACT

A method and an apparatus for producing plastic objects with hollow spaces is disclosed. The method comprises the following steps: establishing a fluid connection (9) between the main cavity (2) of a mold (1) and an overflow cavity (8) and injecting a sufficient amount of plastic melt into the cavity (2) of the mold (1) along a melt flow path, which extends from a plasticizing unit (4, 5) through a plastic injection nozzle (3) into the mold (1), such that plastic melt passes from the main cavity (2) into the overflow cavity (8). The fluidic connection (9) between the main cavity (2) and the overflow cavity (8) is then broken-off prior to the introduction of a pressurized fluid, especially a pressurized gas, into the melt in the cavity (2) by means of at least one fluid injection nozzle (7), so that the melt situated in the cavity (2) is distributed in the cavity (2) while forming a hollow space, and is pressed against the cavity walls of the mold (1). The thus formed molded part is then allowed to cool down to a temperature below the melting point of the plastic melt, the cavity pressure is relieved, and the molded part is removed from the mold. The method advantageously achieves the result that the produced molded parts do not have any gas exit openings and the waste in the overflow main cavity (8) remains minimal.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 5,098,637 to Hendry discloses an internal gas pressuremethod that is used to produce hollow plastic molded parts. According tothis prior art technique, a pressurized gas is injected into the plasticmelt within the main molding tool cavity in order to form a hollow spacein the interior of the molded parts. An overflow cavity is connected tothe main cavity of the tool to receive any excess plastic displaced bythe gas. The following process sequence is specified: plastic meltinjection into the tool cavity; expelling a portion of the melt from themain cavity into the overflow cavity, by the introduction of thepressurized gas into the cavity;

cooling the melt; venting the cavity; and the removal of the finishedmolded part from the molding tool.

European Pat. Document EP 0 321 117 B1 to Melea Limited proposes asimilar method. Plastic melt is first placed into the cavity of themolding tool. Then pressurized gas is injected into the melt, whichdistributes the melt over the interior surfaces of the molding tool andforms the hollow space within the melt.

Then follow the steps of cooling, venting, and removal from the mold. Aportion of the melt is expelled from the main cavity into an overflowcavity while the melt or gas is being introduced into the main cavity,such that the overflow cavity first receives the gas, from which the gasthen spreads into the main cavity.

A commonality between the two methods is that the main cavity and theoverflow cavity are connected by a channel. This channel establishes apermanent fluidic connection between the two cavities.

The technology is developed further in German Pat. Document DE 39 13 109C2 to Klochner Ferromatik. Here, the main cavity and the secondarycavity are connected through a channel which contains a valve, whichmakes it possible to disable or enable the connection between the mainand secondary cavities. The corresponding method runs as follows. First,plastic melt is introduced into the mold cavity. The cavity is filledcompletely with the melt.

After the plastic melt has begun to solidify at the walls of the cavity,the core of the plastic body, which is still liquid, is expelled by apressurized gas into the overflow cavity.

A similar method is known from the European Pat.

Document EP 0 438 279 A1 to Keter Plastic. The objective there is toperform the actual injection molding process under the high pressurethat is typical for injection molding, and not under the comparativelylow gas pressure that normally prevails with the interior gas pressuretechniques. A seal is here provided at the end of the cavity, and thisseal is initially closed. In this operating mode, the melt is injectedinto the cavity at high pressure, until the cavity is completely filled.After the melt begins to solidify, the seal is opened, and thus aconnection to an overflow cavity is enabled. The liquid, melted core ofthe molded part is then expelled into this overflow cavity bypressurized gas.

SUMMARY OF THE INVENTION

The previously-described methods have a number of disadvantages. TheU.S. Pat. No. 5,098,637 and EP 0 321 117 B1 specify that the melt isexpelled from the main cavity into the overflow cavity by injectingpressurized gas into the main cavity or into the interior of the meltsituated in the main cavity. This requires expensive control systems forthe gas flow and/or pressure in order to prevent that gas from alsobeing driven from the main cavity into the secondary-cavity, i.e.,breaks-through. If this happens, the finished molded part will have anundesirable opening, i.e., gas channel, which must then be closed in afurther step, further increasing cost by slowing production.

The methods described in the DE 39 13 109 C2 and the EP 0 438 279 A1require that the cavity be completely filled with melt and, before meltis expelled into the overflow cavity, that the melt be allowed topartially solidify, which requires an undesirable waiting time.

The invention concerns an improved method and an associated apparatus inwhich undesirable openings at the surface of the molded part can bereliably prevented. Furthermore, the invention achieves this advantagein the context of a faster production process that eliminates waitingtimes after introduction of the melt and before injection of thepressurized gas.

In general, according to one aspect, the invention features a plasticproduction process in which a fluid connection is established betweenthe main cavity of a mold and an overflow cavity.

A plastic melt is then injected into the cavity of the mold along a meltflow path that extends from a plasticizing unit through a plasticinjection nozzle into the mold. A sufficient amount of plastic is usedsuch that plastic melt passes from the main cavity into the overflowcavity.

The fluidic connection between the main cavity and the overflow cavityis then broken off.

A pressurized fluid, especially a pressurized gas is introduced, intothe melt in the cavity by means of at least one fluid injection nozzle,so that the melt that is situated in the cavity is distributed in thecavity while also forming a hollow space, and is pressed against thecavity walls of the mold.

The molded part is then allowed to cool down to a temperature below themelting point of the plastic melt, and the pressure of the pressurizedfluid within the cavity is relieved; and the molded part removed fromthe mold.

In contrast to known methods, in which the melt is always expelled fromthe main cavity into the overflow cavity by introducing compressedfluid, the invention pursues the path that the transfer of melt from themain cavity into the secondary cavity is initiated exclusively byintroducing the plastic melt into the main cavity and not by injectingpressurized gas. In the invention, the pressurized gas is introducedimmediately after the fluidic connection between the main cavity and thesecondary cavity has been shut off, so that no waiting time is neededbefore injecting the gas. Nevertheless, the melt still reaches allpoints of the mold, including in particular all the corners that are farremoved from the point where the melt is injected into the cavity.

The valve(s) is activated to establish and break-off the fluidicconnection between the main cavity and the overflow cavity.Alternatively, the connection can be effectively broken by controllingor regulating the pressure relationships between the cavity and/or inthe overflow cavity. In this case, no separate elements such as valvesor slides, are necessary but the flow of melt from the main cavity intothe secondary cavity is achieved solely by creating the appropriatepressure conditions.

A further modification of the invention advantageously provides that,when pressurized fluid is introduced into the main cavity, melt isforced back from the cavity through the runner region into theplasticizing unit.

In order to assure that any valve(s) functions when the melt is expelledfrom the main cavity into the secondary cavity, a further modificationof the invention provides that the connection region between the maincavity and the overflow cavity is heated in order to maintain the fluidconnection between the main cavity and the overflow cavity.

The pressurized gas can be introduced into the melt either directly intothe interior of the molded part which is being produced i.e., inarticle, into the region of the plastic injection nozzle, or into theregion of the runner, i.e., in runner.

The apparatus for implementing the method has the following elements: amold comprising at least two halves, with a cavity; a plasticizing unitand a plastic injection nozzle; at least one fluid injection nozzle; andat least one overflow cavity, which has a fluidic connection to the maincavity.

The invention also provides for some way of breaking off the fluidicconnection between the main cavity and the overflow cavity. Valves maybe used or pressure control or regulation used to influence the relativepressure between the cavity and/or in the overflow cavity.

Finally, heater(s) can be provided to heat the connection region betweenthe main cavity and the overflow cavity to maintain a fluidic connectionbetween the main cavity and the overflow cavity when it is initiallyrequested.

The above and other features of the invention including various noveldetails of construction and combinations of parts, and other advantages,will now be more particularly described with reference to theaccompanying drawings and pointed out in the claims. It will beunderstood that the particular method and device embodying the inventionare shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the sameparts throughout the different views.

The drawings are not necessarily to scale; emphasis has instead beenplaced upon illustrating the principles of the invention. Of thedrawings:

FIG. 1 schematically shows the inventive injection molding apparatus forproducing hollow plastic objects according to the invention; and

FIG. 2 is a process diagram illustrating the inventive method.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an injection molding apparatus that has been constructedaccording to the principles of the invention. A mold 1, including atleast two parts, has a cavity 2 or hollow space that defines the outersurface of a molded part 6 that is being produced. A plasticizing unit4, 5 is connected to the mold. It has an extruder barrel 4 and a screw5, which can execute both rotational and axial motion in the barrel. Theconnection between the mold 1 and the plasticizing unit 4, 5 isestablished by a plastic injection nozzle 3.

Two fluid injection nozzles 7 are translated along their axes byactuators so that the nozzles can be moved into and out of the cavity 2by a controller 15 as illustrated by the arrows. They are shownpositioned in the cavity. The pressurized fluid, specificallypressurized nitrogen from a source or supply unit (not shown), isintroduced into the cavity through the nozzles also under control of thecontroller 15. The pressurized gas distributes the melt into the cavity2 and presses it against the walls of the cavity 2. The fluid injectionnozzles 7 can be attached at arbitrary points of the mold. For example,only one nozzle 7 can be used which is disposed concentric with theplastic injection nozzle 3. In this way, melt and fluid are introducedthrough the same mold opening.

In place of fluid injection nozzles which extend directly into thecavity, other known gasification elements can alternatively oradditionally be used such as fluid nozzles that are integrated into theplastic injection nozzle 3 or into the runner. It may also be desirableto use only one fluid injection nozzle 7 in some cases.

In the present case, two overflow cavities 8 are connected to the maincavity 2. These receive the plastic melt via respective a fluidicchannels 9 which extend between the main cavity 2 and the overflowcavities 8. Each channel 9 is equipped with heaters 11, which preventthe melt from cooling in the connection channels, so that it does notfreeze and plug-up the connection channel. The heaters 11 thus assure aconstant fluidic coupling between the main cavity and the secondarycavities, if this is desired in connection with this method. Valves 10are usually used in the channels 9 to regulate the flow of plastic.

Alternatively, the connection channel 9 also can be designed to be veryshort, so that it actually is omitted; the main cavity 2 and theoverflow cavity 8 are then connected directly to each other. In thisconfiguration, valve 10 is always disposed in the connection channel 11to block for the flow of melt from the main cavity 2 into the overflowcavity 8.

FIG. 2 shows the process for producing a molded part 6.

First in step 210, the connection channel 9 between the main cavity 2and the overflow cavities 8 is released by opening the valves 10, sothat melt can flow over from the main cavity 2 into the secondarycavities 8 by the controller 15. A fluidic connection between the maincavity 2 and the overflow cavities 8 is thus established.

Then in step 212, the plastic melt is injected into the cavity 2 throughthe plastic injection nozzle 3, in an amount that is sufficient toproduce the desired molded part 6. Since the connection channels 9 areopen, the melt passes from the main cavity 2 into the overflow cavities8. It is desirable to locate the overflow cavities as shown so that eventhe remotest point of the cavity from the perspective of the plasticinjection nozzle 3 is also reached by the melt. As a result, all the`corners` of the cavity 2 are filled with melt.

When sufficient melt has been injected into the cavity, the valves 10are then closed to block the connection channels 9 between the maincavity and the secondary cavities so that no more melt can flow over instep 214.

Only after the valves are closed is pressurized fluid introduced in step216 by means of the fluid injection nozzles 7, which have been locatedin the interior of the melt in the cavity 2 by actuators. FIG. 1 showsthis state of the process. The introduction of pressurized fluid resultsin the formation of hollow spaces in the interior of the melt. Thesehollow spaces are beneficial, on the one hand, because of weight savingsin the molded part 6 compared to a solid part. Furthermore, sink markson the surface of the molded part 6 that originate from the volumecontraction of the melt during the cooling process are also prevented.

If larger hollow spaces are desired in the interior of the molded part 6that are greater than the volume contraction due to cooling, it can beadvantageously arranged that melt is pressed through the runner region12 back into the plasticizing unit 4, 5 in step 218. The screw 5 canhere be withdrawn - with force pre-tension--by an amount thatcorresponds to the melt volume that is to be returned. In contrast tothe connections between the main cavity 2 and the overflow cavities 8,no valves 10 are provided between the cavity 2 and the runner region 12or the screw ante-chamber.

In the prior art, melt is expelled into the overflow cavity generally byintroducing gas (which disadvantageously increases the waste volume inthe overflow cavities). By contrast, the inventive method makes itpossible to return this volume of melt, at least partly, directly to theplasticizing unit. As a result, this waste is not created. Rather, thisvolume will be processed immediately at the next shot for the followingmolded part.

When the melt, which forms the molded part 6 has cooled sufficiently, itcan be removed from the mold 1.

However, before the two halves of the mold are opened, the fluidpressure in the molded part must first be dissipated in step 220. Forthis purpose, the gas flow path is enabled, e.g.. by means of suitablevalves, which are not shown here.

Before the next injection molding cycle, the overflow cavity 8 must alsobe emptied in step 222. Its solidified contents can be sent forrecycling.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and detail may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

I claim:
 1. A method for producing plastic parts with hollow spaces,comprising:establishing a fluid connection between a main cavity of amold and an overflow cavity; injecting a sufficient amount of plasticmelt into the main cavity of the mold along a melt flow path, whichextends from a plasticizing unit through a plastic injection nozzle intothe mold, such that plastic melt passes from the main cavity into theoverflow cavity; breaking off the fluid connection between the maincavity and the overflow cavity; after the fluid connection between themain cavity and overflow cavity has been broken off, introducing apressurized fluid into the melt in the main cavity via at least onefluid injection nozzle to distribute the plastic melt situated in themain cavity while forming a hollow space; allowing the produced moldedpart to cool down to a temperature below the melting point of theplastic; relieving the cavity of the pressure of the pressurized fluid;and removing the molded part from the mold.
 2. The method of claim 1,further comprising activating at least one valve to establish and breakoff the fluid connection between the main cavity and the overflowcavity.
 3. The method of claim 1, further comprising regulating pressureconditions in the main cavity and/or in the overflow cavity to establishand break off the fluid connection between the main cavity and theoverflow cavity.
 4. The method of claim 1, further comprising forcingmelt back from the main cavity through a runner region into theplasticizing unit by introducing the pressurized fluid.
 5. A method forproducing plastic parts with hollow spaces, comprising:establishing afluid connection between a main cavity of a mold and an overflow cavity;injecting a sufficient amount of plastic melt into the main cavity ofthe mold along a melt flow path, which extends from a plasticizing unitthrough a plastic injection nozzle into the mold, such that plastic meltpasses from the main cavity into the overflow cavity; heating aconnection region between the main cavity and the overflow cavity tomaintain the fluid connection between the main cavity and the overflowcavity: breaking off the fluid connection between the main cavity andthe overflow cavity; introducing a pressurized fluid into the melt inthe main cavity via at least one fluid injection nozzle to distributethe plastic melt situated in the main cavity while forming a hollowspace: allowing the produced molded part to cool down to a temperaturebelow the melting point of the plastic: relieving the cavity of thepressure of the pressurized fluid: and removing the molded part from themold.
 6. The method of claim 1, further comprising introducing thepressurized fluid directly into the interior of the molded part that isbeing produced.
 7. The method of claim 1, further comprising introducingthe pressurized fluid in the area of the plastic injection nozzle or inthe area of a runner.
 8. The method of claim 1, further comprisingestablishing the fluid connection between the main cavity and theoverflow cavity prior to injecting the plastic melt into the maincavity.